A thermoplastic cellulose derivative composition of the present invention contains, as a main component, a cellulose ester having an aliphatic polyester side chain having a repeat unit having 2 to 5 carbon atoms, wherein a rate of heating loss at 200null C. is 5 wt % or less, a melt viscosity at 200null C. and 1000 secnull1 is 50 to 300 Panullsec, and a melt tension at the time of take-up at 200null C. and 100 m/min is 0.1 to 40 mN. The present invention can provide excellent fiber products by melt spinning of the composition.